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ME - EXOSAT ME Spectra and Lightcurves


The EXOSAT Medium Energy experiment (ME) was an array of eight proportional counters with a total geometric area of 1600 cm^2. The field of view was square and 0.75 degrees x 0.75 degrees FWHM on each side. Each proportional counter consisted of an argon chamber on top of a xenon chamber separated by a 1.5 mm beryllium window. The fractional energy resolution, dE/E, was 21(E/6 keV)^-0.5 percent FWHM for the argon chambers. The output from each chamber was pulse height analyzed into 128 channels with the argon chambers sensitive from 1-20 keV and the xenon from 5-50 keV. To optimize the background subtraction, each half of the detector array was alternately offset to a source-free region of sky to monitor the particle background. Only results from the argon detectors are included in the EXOSAT ME database.

The high count rates given by the ME required OBC (on board computer) programs to compress the data prior to their being telemetered. Depending on the objective of the observation the OBC programs traded time resolution against spectral information. Depending on the telemetry load, and the OBC programs running for the other two experiments, two or three ME programs could be run simultaneously. The spectral orientated programs gave spectra plus intensity profiles. The timing programs gave purely intensity profile data with in some cases selectable channels. The highest time resolution possible for a single selectable energy band was 0.2 ms.

The products available within this database has been created using the data sampled by the spectral orientated OBC programs.


Turner M.J.L., Smith A. and Zimmermann H.U. 1981, Sp. Sci. Rev., Vol. 30, p. 513.

Data Products

The available products originally created at the EXOSAT observatory have been converted in FITS format at HEASARC. The data products available include lightcurves in one and two energy bands with a time resolution of a few seconds, plus an average spectrum with the associated response matrix for each observation. A plot of the lightcurves and spectra are also provided as GIF file. For high time resolution studies it is still necessary to return to the original EXOSAT data tapes (FOTs). These can be obtained either from the on-line HEASARC service FTP area or sending a request at the EXOSAT Project Scientist, Astrophysics Division, ESTEC, 2200 AG Noordwijk, The Netherlands.

Spectra: For each observing interval there is a background-subtracted on-source spectrum integrated over the entire observing interval. The background subtraction method to derive the on source spectra depends on the mechanical configuration of the ME array detector used during an observation. The quality of these spectra is primarily determined by how good the background subtraction was. This is indicated by the QFLAG parameter which has a range from 1 to 5. Spectra with a QFLAG less than 3 are usually not of sufficient quality and should not be used. pTo each entry in the database is associated a spectral file in FITS format and a plot of the FITS data in GIF format.

Lightcurves: Typically during an EXOSAT observation, one or a combination of the following mechanical ME configurations were used:

 * only one half of the ME array was on source
   and the other half monitoring the background throughout an observation

 * both half on source

 * the two halfs alternatively were monitoring the source and the
   background swapping after few hours.
For each mechanical configuration interval the following type of lightcurves in FITS format are available:
 * On-source background-subtracted 30 s light curves in the energy
   range 1-8 keV. The filename of these lightcurves starts
   with the suffix 'D'.

 * Background light curves in the energy range 1-8 keV with a time
   resolution of 30 s (non available for co-aligned configuration).
   The filename of these lightcurves starts with the suffix 'R'.

 * Multiband (1-10 s) time resolution on-source background-subtracted light
   curves in the energy bands 1-3.8, 3.8-8 and 1-8 keV. The filename of these
   lightcurves starts with the suffix 'A', 'B' and 'C' respectively. The time
   resolution of these files is the maximum available from the pulse height
   analyzed (spectral) data and depends on that specified by the principle
Lightcurves were accumulated everytime a new mechanical configuration occurred during an observation interval and therefore one or more FITS lightcurves, for each type, can be associated to a database entry.

The lightcurves are also provided as GIF files. To each entry in the database are associated two GIFs. One shows the full energy band (0.8-9 keV 'D' files), the other shows the lightcurves for the 0.8-3.6 keV band and 3.6-9 keV band (the 'A' and 'B' files). The lightcurves plotted in the GIF file are obtained using all the files associated to that database entry.

Automatic Analysis

The ME auto was designed to cope with the myriad of OBC modes, observing configurations and types of sources observed by the ME. Because of the complex operation of this instrument, a second pass through some 20 to 30 percent of the observations was required to "fix" problems, primarily due to poor background subtraction. This was completed in mid-1990. Even so, the quality of the spectra and lightcurves is dependent on the method of background subtraction used and on the stability of the background. For many observations the automatic analysis gives acceptable spectra for sources as faint as 2.0 ct/s/half (approx. 1 UFU). The limiting sensitivity for a source detection is 0.2 ct/s/half (approx. 0.1 UFU).

The ME auto-processed only the argon chamber data. It proceeded as follows:

 * Selection of the method of background subtraction, which depends on the
   set up of the ME configuartion during the observation.

 * Creation of spectra and lightcurve files.

 * Spectral and timing analysis.
The selection of the most appropriate method of background subtraction for a particular observation is the crucial step in determining the quality of the data products and results. The auto chooses in a hierarchical manner from the following list of available background subtraction techniques, using:
 * Method 1: the generation of difference spectra (SWAP3A), only possible
             for observation in which the two halfs of the ME array
             alternatively were monitoring the source and the background
             swapping after few hours.

 * Method 2: the standard difference spectra (SWAP1,SWAP2,SWAP2A or SWAP3B),
             possible only as in Method 1 a swap is performed during
             an observation.

 * Method 3: slew spectra (SLEW1 or SLEW2), typically used when
             the same half or both half are on source thoughout an

 * Method 4: standard backgrounds (NOSLEW), used only if slew data
             are not available or contaminated by a source.
Methods 1 and 2 generally produce the best results because background data is obtained from the same detectors at a different time when they were offset to view a source-free region of sky. The major uncertainty in these cases is due to a difference in the background spectrum and counting rate between the aligned and offset positions. This difference is removed using "difference spectra".

In method 1 the array swaps were done in a sequence where the difference spectra can be uniquely determined from +ve and -ve offset background spectra (see EXOSAT Express No. 16, p. 21). This usually gives the best background subtraction possible. If the set of array swaps was incomplete then method 2 uses standard difference spectra generated from background observations. If no suitable array swap data are available then method 3 uses background spectra obtained during the slew on to or off from (or both) the source. If no suitable slew data are available, then method 4 uses standard background spectra. Method 4 usually produces satisfactory results only for bright sources.

If the background subtraction was unsatisfactory after the first pass a number of "fixes" were tried:

 * Time windows were used to exclude times when the background was

 * If satisfactory background subtraction cannot be obtained using methods
   1 and 2 then the same macros are rerun using only the corner detectors.
   These have very small (or zero) difference spectra. If this procedure has
   been followed it can be, when plotting a spectrum, that dets = 10011001.

 * If one detector appears to be significantly noisier than the others, then
   it is excluded from the analysis.

 * If method 4 has been used for background subtraction (NOSLEW) then an
   attempt is made to find a more suitable "standard" background. The default
   backgrounds are averaged over the whole mission and sometimes better
   backgrounds can be found from nearby observations.
In addition, a number of other problems were dealt with:
 * Occasionally there was a temporary loss of spacecraft pointing during an
   observation. These intervals are excluded from the auto.

 * Sometimes the automatic software would assume that the pointing position of
   the previous observation applied to the current one. This is corrected by
   cross-correlating the results database with the EXOSAT log.

 * Any bursts (type I only) are removed from the spectra (but not the
   light curves). A comment is then added to the spectrum  to indicate
After selecting the background subtraction method and fixing the other problems a spectrum was created integrated over the entire observation from all detectors. In addition, light curve files are created at the highest time resolution available in the spectral telemetry stream; this typically ranges between 1 and 10 s, although on occasion higher time resolution was available. Three files are created with light curves covering the energy bands of 1--3, 3--8 and 1--8 keV for the source and 1--8 keV for the background data.

If the source count rate is > 0.25 ct/s/half, then automatic spectral fitting and timing analysis was carried out, with the results written to the database.


The quality checking concerns the reliability of the background subtraction. All spectra have quality flags ranging from 0 to 5 (unusable to excellent) defined below:
                Qflag              Meaning

                  0                unusable
                  1                very poor
                  2                poor
                  3                acceptable
                  4                very good
                  5                excellent
Generally, spectra with quality flags of 3 or more are suitable for spectral analysis while those with quality flags of 0, 1 or 2 have major problems and should only be used with extreme caution. In many cases it may be possible to improve the quality of the background subtraction by returning to the FOTs and carrying out a more complex analysis.

There are a number of points that the user should carefully check before using ME auto products and results.

The ME quality flag that is assigned to each entry is a subjective measure of how well the background subtraction has been done. It does not take into account the possibility of there being more than one source within the 45 arc minute ME field-of-view --- such entries can still have good quality flags and it is up to the user to determine from where the observed flux originates.

Another potential problem is the presence of weak contaminating X-ray sources in the offset detectors. Their effect will depend on the direction of any array swaps and the background subtraction method used, but will generally cause the background to be over-subtracted. Plotting the background counting rate `pp/bg` is a good way of checking for this.

When plotting the background counting rates from the offset detectors there will always be a step up or down in counting rate at the time of an array swap. This occurs because the background is slightly different in the two detector halves.

Lastly, after a slew on to a source there was always an interval of usually a few minutes, but sometimes up to an hour, before the pointing control was switched to the star trackers. When this occurs the pointing sometimes changed by several arc minutes and this can cause a step up in counting rate. These should have been removed in the quality checking, but it is possible that some have been missed.

The spectral parameters obtained from the automatic spectral fitting should be used with great caution. Automated spectral fitting programs are far from infallible and in many cases may have gone astray. The following problems may arise:

 * there may be more than one model that gives an acceptable fit

 * the automatic spectral fitting program sometimes was trapped in a
   false minimum, causing apparently no model, or the wrong model, to
   give the best fit. This occurs most often for models 6 -- 8.

 * entries with `Qflag ME` <3 indicate problems with background subtraction,
   which may affect the spectral fit.
Always extract the spectrum and fit again using XSPEC.


On 14 May 1990, the final entries were added to the ME database, resulting in a total of 2291 entries. There are still a small number of missing observations. These are:
 * All observations after day 77 of 1986. After this time the EXOSAT
   pointing was not stable and it is necessary to apply a
   time-dependent collimator correction to light curves and spectra.

 * Some of the very early observations where there is no useful
   background spectrum.


ASPEC Summary File exists?

Chi squared for BB

Temperature for BB

NH for BB

Norm for BB

Beta Angle (degrees)

The number of the available background lightcurves is stored in BG_FILES. The file name of the lightcurves is composite by a character, which for the background lightcurve is 'R', and a 5 digits number. The parameters ROOT_4, ROOT_5, and ROOT_6 contains the values for the 5 digits number present in the file name of the background lightcurves. NOTE not always a background lightcurve is available. This is because not always there were detectors monitoring the background during an observation.

The galactic latitude of the source.

Chained? *=start, L=continuation

Reduced Chisq

Classification flag

Alpha for Co

Chi squared for Co

Temperature for Co

NH for Co

Norm for Co

The count rates stored in the ME database and in the spectra are normalized to a half array. They are NOT corrected for collimator transmission. Normally this is a small (5 percent) effect unless the pointing or source positions were in error.

In the case of the light curves, the count rates have been corrected for collimator transmission using the source position given by parameters `RA` and `Dec` and spacecraft pointing position in `point RA` and `point Dec`.

Other count rates and associated errors are

   * `Rate 1` - The count rate in band 1, 1-3 keV
   * `Rate 2` - The count rate in band 2, 3-6 keV
   * `Rate 3` - The count rate in band 3, 6-10 keV
   * `Rate 4` - The count rate in band 4, 10-15 keV
   * `Rate total` - The count rate in the entire available band


The Declination of the target.

The end time of the observation. The units and use of this parameter are the same as those for `time`.

Line Energy

EW of line

Expected Variance 10s

Expected Variance 1-3 keV

Expected Variance 1-8 keV

Expected Variance 3-8 keV

Expected Variance BG

Expected Variance int*10

The `exposure` is the total on-source observation time in seconds. This includes all dead time effects, interruptions in coverage, etc.

File name of the GIF file which shows a plot of the two lightcurves in the 1-3.8 and 3.8-8 keV band.

File name of the GIF file which shows a plot of the lightcurve in the 1-8 keV band.

Root name for lightcurves.

This parameter contains the file name of the associated spectrum.

Hardness ratio (6-10 keV / 3-6 keV) and error.

Error in hardness ratio.

Source order number.

MJD of creation time.

The number of file for each sets of lightcurves associated to the database entry is stored in the parameters LIGHTCURVE_NUM. There are 4 different sets of lightcurves (see also data products). The file name of the lightcurves is composite by a character and a 5 digits number. The character have 4 values 'D', 'A', 'B', 'C', each corresponding to a set of lightcurves (see also description in data products). The parameters ROOT_1, ROOT_2, and ROOT_3 contains the values for the 5 digits number present in the file name of the lightcurve.

The galactic longitude of the source.

1=line, 0=no line in fit

Char length of lightcurve name

Char. length of spectrum name

Macro completed OK ?

MACRO_NAME contains the name of the macro used in the Automatic Analysis. See the description of the macro in Automatic Analysis.

The `name` parameter gives the target name. This name usually corresponds to the target name as specified by the original observer. If a source has been observed by different observers it may be entered under two or more names. It is always recommended that any search by `name` be followed by a search on the found coordinates.

Galactic NH

Number of on-source lightcurves

# of spectra associated. w/entry

Number of parameters

Observed Variance 10s

Observed Variance 1-3 keV

Observed Variance 1-8 keV

Observed Variance 3-8 keV

Observed Variance BG

Expected Variance int

Observed Variance int

Observed Variance int*10

Best fit model parameter 1

Best fit model parameter 10

Best fit model parameter 2

Best fit model parameter 3

Best fit model parameter 4

Best fit model parameter 5

Best fit model parameter 6

Best fit model parameter 7

Best fit model parameter 8

Best fit model parameter 9

In PACKET_ID is stored the value of the OBC data stream using to create the products.

PI Code

Alpha for PL

Chisq for PL

Nh for PL

Norm for PL

name to be printed inc. blanks

The declination of the EXOSAT pointing.

The RA of the EXOSAT pointing.

MJD of macro run

Proposal Code

Observation Type

0 is unusable, 1 is very poor, 2 is poor, 3 is acceptable, 4 is very good and 5 is excellent.

The RA of the target.

Counts/s/4dets band 1, 1-3 keV

Error/s/4dets band 1, 1-3 keV

Counts/s/4dets band 2, 3-6 keV

Error/s/4dets band 2, 3-6 keV

Cts/s/4dets band 3, 6-10 keV

Error/s/4dets band 3, 6-10 keV

Cts/s/4dets band 4, 10-15 keV

Error/s/4dets band 4 10-15 keV

Counts/s/4dets in total band

Error/s/4dets in total band

RMS Variability 10s

RMS Variability 10s error

RMS Variability 1-3 keV

RMS Variability 1-3 keV error

RMS Variability 1-8 keV

RMS Variability 1-8 keV error

RMS Variability 3-8 keV

RMS Variability 3-8 keV error

RMS Variability BG

RMS Variability BG error

RMS Variability int

RMS Variability int*10

RMS Variability int*10 error

RMS Variability int error

Roll Angle (deg*10)

Root 1 name

Root 2 name

Root 3 name

Root 4 name

Root 5 name

Root 6 name

Total No. of root names

Sequence Number

Best Fit model code

Softness ratio (1-3 keV / 3-6 keV) and error.

Error in Softness ratio

File Number

TFTSA Summary File exists?

Chi squared for TH

Temperature for TH

NH for TH

Norm for TH

The `time` of the observation refers to the start time. Times are accurate to the nearest second.

The time resolution of the intensity data in seconds.

Total # of roots w/same seq #

The time resolution of the spectrally resolved data, in seconds.

Contact Person

Questions regarding the ME database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk